Bone grafting is widely used to treat fractures, non-unions and to induce arthrodeses. Autogenous cancellous bone, which is taken from one site in the graftee and implanted in another site in the graftee, is currently the most effective bone graft. Autogenous cancellous bone provides the scaffolding to support the distribution of the bone healing response. Autogenous cancellous bone also provides the connective tissue progenitor cells which form new cartilage or bone. However, the harvest of autogenous bone results in significant cost and morbidity, including scars, blood loss, pain, prolonged operative and rehabilitation time and risk of infection. Furthermore, in some clinical settings, the volume of the graft site can exceed the volume of the available autograft. Accordingly, alternatives to autografts have been developed in an attempt to reduce the morbidity and cost of bone grafting procedures.
Several purified or synthetic materials, including ceramics, biopolymers, processed allograft bone and collagen-based matrices have been investigated or developed to serve as substitutes for autografts. The FDA has approved a porous coral derived synthetic hydroxyapatite ceramic for use in contained bone defects. A purified collagen/ceramic composite material is also approved for use in acute long bone fractures. Although these materials avoid the morbidity involved in harvesting autografts from the graftee and eliminate problems associated with a limited amount of available autograft, the clinical effectiveness of the synthetic materials remains generally inferior to autografts.
The synthetic graft materials have also been used as carriers for bone marrow cells. When such composite materials have been implanted into skeletal defects, the connective tissue progenitor cells differentiated into skeletal tissue. In some instances, the composite implants were made by soaking the synthetic graft material in a cell suspension obtained from a bone marrow plug. However, the connective tissue progenitor cells, which have the capacity to differentiate into cartilage, bone and other connective tissue such as fat, muscle, and fibrous tissue are present in the bone marrow in very minute amounts. The numbers of such cells present in 1 ml of bone marrow varies widely from subject to subject from about 100 cells to 20,000 cells. This represents a mean of about one in 20,000 to one in 40,000 of the nucleated cells in bone marrow. Thus, a composite implant made by soaking a given volume of synthetic carrier graft material in a comparable volume of fresh bone marrow contains relatively few connective tissue progenitor cells.
Accordingly, a technique has been previously developed to increase the relative concentration of connective tissue progenitor cells in composite implants. This technique involves plating a suspension of bone marrow cells onto tissue culture dishes, culturing the cells in a select medium for one or more days until the number of connective tissue progenitor cells in the culture increases, and then detaching the cells from the tissue culture dishes to provide a cell suspension containing a culturally-expanded population of connective tissue progenitor cells. Composite implants are then made by soaking synthetic ceramic carriers in this suspension of culturally-expanded cells. Unfortunately, this method of preparing composite implants is very time consuming. Moreover, if the culturally-expanded cells used in this method are derived from bone marrow aspirates obtained from the graftee, the graftee must undergo multiple invasive procedures, one to remove his or her bone marrow and one at a later date to implant the composite implant. In addition, the graftee may be exposed to anaesthesia more than once.
Accordingly it is desirable to have a new method of preparing a composite bone marrow graft which can be performed intraoperatively, i.e., at the same time bone marrow is being taken from the graftee. An intraoperative method of preparing a composite bone marrow graft which uses bone marrow aspirate as the source of the connective tissue progenitor cells and which results in the formation of a composite bone graft containing an enriched population of connective tissue progenitor cells is especially desirable.